Surface mounted devices are small leadless components that are being used increasingly in the assembly of electronic circuitry by robotic equipment. To facilitate their handling by automated assembly equipment, these components are generally provided in carrier compartments on a component dispensing tape. Each carrier compartment is sized to contain a single component. An exemplary tape may be eight millimeters in width and sized to contain components that measure 3 mm by 1.5 mm by 0.5 mm.
Along one side of the component dispensing tape are tractor drive or index holes. The automated assembly equipment generally includes a toothed drive wheel that advances the tape by engagement with these holes. The spacing between the holes corresponds to the spacing between the carrier compartments so that when the tape is advanced one hole, a single component is advanced, usually to a robotic assembly arm for placement on a circuit board.
Sometimes the component carrier compartment advanced to the robotic assembly arm is empty. Empty carrier compartments in a tape may be the result of a variety of causes, but can most often be traced to the process by which the components are loaded into the carrier compartments by the component supplier. In order for the automated assembly equipment to function properly despite its occasional encounters with empty carrier compartments, such equipment is usually provided with a system for detecting whether a component carrier compartment is full. Typically, this system monitors the pressure in a vacuum probe that is used to pick up components for placement on the circuit board. When the vacuum increases, the probe has successfully lifted a component. If the equipment finds that a carrier compartment is empty, it causes the toothed drive wheel to advance the tape by another tractor drive hole in an attempt to provide a component to the robotic assembly arm.
The component dispensing tape is generally provided in reel form, with a single reel having approximately 10 to 25 feet of tape. Depending on the density of component carriers in the tape, such a reel may contain thousands of components.
Prudent principles of inventory management dictate that thousands of components should not sit idly at an assembly station, waiting, sometimes for a year or more, to be used in the assembly of a circuit board. Certain surface mounted components, such as integrated circuits, are relatively costly, so that a full reel of components may cost in excess of $10,000. In order to better manage component supply and demand, it is desirable to provide each assembly station with only the number of components that it is expected to use in a certain product run or in a certain period of time.
In the prior art, the length of tape needed to supply a desired number of components to an assembly station was determined manually. Since counting each of the desired number of components was impractical, the number of components in a short length of the tape was counted instead. The length of tape required for a particular assembly run was then estimated based on the component density of the sampled length of tape, taking into account the occurrences of empty carrier compartments in the sampled tape. The desired length of tape could then be unrolled, measured and cut. Although somewhat wasteful of components, the tape would generally be cut slightly longer than the computed length to ensure that the equipment did not prematurely exhaust its components supply before the assembly run was completed.
This prior art method is cumbersome and poorly suited for use with today's highly automated circuit board assembly techniques. The prior art method is also inaccurate. The component density in a short length of tape is not necessarily indicative of the component density throughout the reel.